1. Field of the Invention
The present invention relates to improved carbon black for tire tread rubber. More particularly, the present invention relates to carbon black which imparts upon tire tread rubber improved treadwear, lower rolling resistance, lower heat buildup and improved tear resistance. The improved carbon black is in the N100 series and, when in rubber, combines the improved treadwear and low heat buildup advantages of N121 carbon black with the high tear resistance properties of N115. The present invention has particular application to truck and bus steel cord radial tire treads, especially for use in on/off highway conditions, and for high performance passenger car tires.
2. General Background
As worldwide dependence on truck transportation and other truck tire usage steadily increases, new designs in radial truck tire technology will have an increasingly significant impact on the many economies of the world. In recent years, many radial truck tire performance improvements have been made. However, further advances to resist both premature failure and accelerated treadwear in severe services are still needed in tire design and in compounding materials development. The art has recognized that there is a long felt but unresolved need for improved failure resistance and other performance improvements for tread compounds including longer treadwear, improved tear resistance, lower heat build-up and lower rolling resistance. (See, e.g., M. B. Rodgers and S. M. Mezynski, Kautschuk Gummi Kunst., 46, (9), 718 (1993); and, B. Lambillote and G. S. Eiber, Rubber World, 209, (1), 27 (October 1993)).
Carbon black, a form of elemental carbon, is widely used as a component of tire rubber, for both natural and synthetic rubbers and blends of natural rubber with synthetic polymers. The physical characteristics of carbon black, such as particle size and structure, affect various performance properties of rubber compounds, such as tire treadwear, rolling resistance, heat buildup and tear resistance.
The present invention relates to a new carbon black (herein referred to as xe2x80x9ccarbon black Axe2x80x9d) designed to improve the qualities of tire tread rubber, including providing improved treadwear, lower rolling resistance, lower heat buildup and improved tear resistance. The improved carbon black is in the N100 series and, when in rubber, combines the long treadwear and low heat buildup advantages of N121 carbon black with the high tear resistance properties of N115. The carbon black of the present invention is particularly well suited for improving the qualities of tread for truck and bus steelcord radial tires (TBS/RT), and high performance passenger car tires. Medium or heavy-duty steelcord radial truck and bus tires (TBS/RT) encompass a number of market segments classified as over the highway truck and bus, construction/agricultural, mixed on/off road, city service and special fuel economy tires.
Currently, N100 and N200 series tread carbon blacks (as specified in ASTM D 1765) are used in TBS/RT and their retread compounds. These tread blacks are broadly characterized as having both high surface area and high structure levels.
The structure of a carbon black is the degree of particle aggregation, with a high structure carbon black having more particles aggregated into random structures than a low structure black. The structure of carbon black can be defined by the n-Dibutyl Phthalate Absorption number (DBPA). The larger the DBPA number, the higher the structure of the carbon black. Surface area can be measured by Iodine Adsorption number (Iodine No.). There is an inverse relationship between the iodine number surface area and particle size; the higher the number, the smaller the particle size.
The particle size and structure of carbon black affect various qualities of rubber containing carbon black, such as tire treadwear, tread rolling resistance, tread heat buildup and tread tear resistance. Accordingly, different carbon blacks are used in different treads depending on the specific service requirements of the tires.
For example, on-highway truck service exposes tires to high loads and high speeds over relatively smooth roads. For this application, in both Europe and North America, tread blacks such as N121, N110 and N234 are mainly used. Here, the performance concerns relate primarily to longer and more even treadwear.
However, rolling resistance is becoming a very important performance concern worldwide. In TBS/RT the tread plays the most important role in controlling rolling resistance.
The three aforementioned tread blacks (N121, N110 and N234), exhibit relatively high hysteresis characteristics (developing higher heat build-up temperatures and higher rolling resistance levels relative to coarser N200 and N300 series counterparts), with N110 being the most hysteretic followed by N121 and N234. Relative to each other, N121 develops the highest treadwear rating, followed by N234, and then N110. The applications for these tread blacks include new tread and retread compounds.
In on/off-highway truck service, the tires, when off the highway, generally experience rougher roads and sharper turns with high loads and at lower speeds than on-highway service. However, when they come back to the highway they experience the same high speeds and temperature conditions as on-highway tires.
For Europe and North America, the tread blacks commonly employed for this application include N110, N115 and N220. The performance concerns relate more to service life of the tread than to treadwear life. The greater the severity of tire service conditions the more important the resistance to failure becomes, particularly in the tread compound. These grades develop lower modulus levels and exhibit more resistance to tear than N121 or N234. N110 and N115 develop higher heat build-up temperatures and rolling resistance levels than N220 (or N121 and N234) and higher tear resistance levels.
While N115 and N110 are used in on/off highway treads in Europe, N115 is not used as much in this application in North America. These two tread blacks differ mainly in that N115 has higher tint, iodine number and nitrogen surface area levels than N110. Nevertheless, they perform similarly in rubber.
TBS/RT are often composed of carbon black reinforced polymer systems that are based mostly on natural rubber (NR) or blends of NR and synthetic polymers (emulsion styrene-butadiene copolymer, SBR, and polybutadiene, BR, rubber). (See, e.g., M. B. Rodgers and S. M. Mezynski, Kautschuk Gummi Kunst., 46, (9), 718 (1993), which is incorporated herein by reference)).
NR (natural rubber) is a natural product from latex-producing caoutchouc plants, of which the Hevea Brasiliensis is the most common, is a polyisoprene (methyl butadiene) elastomer.
BR (butadiene rubber) is a synthetic rubber produced from either an emulsion or solution polymerization of butadiene joined mostly linearly by 1,4 (preferred in cis-1,4 but, also in certain measure, trans-1,4 conformation) and by 1,2 additions.
SBR (styrene-butadiene rubber) is a synthetic rubber produced from either an emulsion or solution polymerization of butadiene and styrene in various ratios.
Among the rubbers suitable for use with the present invention are any natural rubbers, synthetic rubbers and blends of natural and synthetic rubbers. These include the so-called diene elastomers, i.e., for example oil-treated natural and synthetic rubbers, such as carboxyl rubbers, epoxy rubbers, transpolypentenamer, halogenated butyl rubbers, rubbers of 2-chlorobutadiene and polybutadiene rubbers. Typical of the synthetic rubbers are styrene-butadiene rubbers (SBR), whether clear or oil extended, emulsion SBR rubbers, high styrene SBR rubbers, solution SBR rubbers, starred solution SBR rubbers and functionalized solution SBR rubbers.
Further still, suitable rubbers are rubbers, plastics and mixtures thereof which can be crosslinked with sulfur and vulcanization accelerator(s) and also with peroxide to form elastomers. These include the so-called diene elastomers, i.e. for example oil-extended natural and synthetic rubbers, such as natural rubbers, terpolymers of ethylene, propylene and unconjugated dienes; copolymers of ethylene and propylene and also carboxyl rubbers, epoxy rubbers, transpolypentamer, halogenated butyl rubbers, rubbers of 2-chlorobutadiene, ethyl/vinyl acetate copolymers and, optionally, chemical derivatives of natural rubber and modified natural rubbers. Any suitable natural or synthetic vulcanizable rubber can be used for purposes of the invention.
Copolymers can be used by themselves or mixed with at least one other diene elastomer, in particular polyisoprene, natural rubber or polybutadiene. The elastomer used in the blend is preferably polybutadiene having more than 90% cis-1,4 bonds obtained by known methods of catalysis with the use of transition metal as described, for instance, in French Patent 1,436,706. This other diene elastomer can be present in variable proportions with respect to the copolymer prepared in solution, and preferably up to 70 parts by weight.
As conjugated diene, there are suitable, in particular, butadiene-1,3, isoprene, and 2,3-dimethyl-1,3-butadiene. As aromatic vinyl compound, there are suitable, in particular, styrene, ortho-, meta- and para-methylstyrene or the commercial xe2x80x9cvinyl-toluenexe2x80x9d mix.
The copolymer of conjugated diene and aromatic vinyl compound prepared in solution should have a total content of styrene of between 5 and 50% by weight and a glass transition temperature (Tg) of between 0 degrees and negative 80 degrees Celsius when measured by differential thermal analysis. The content of vinyl bonds in the thermal butadiene fraction incorporated can be between 20 and 80%, the content of trans-1,4 bonds can be between 20 and 80%, and the content of cis 1,4 bonds is complementary to the contents of vinyl bonds plus trans-1,4 bonds.
NR systems normally provide improved tear strength and building tack over synthetic polymer compositions and lower hysteresis due to the lower internal energy loss exhibited by NR on deformation. Further, the use of BR with NR can impart enhanced resistance to cut propagation and fatigue and improved abrasion resistance. SBR is used primarily in these blends to improve wet traction performance.
Under low severity operating conditions, NR provides the best treadwear resistance performance relative to NR/BR blends. With higher severity highway conditions, NR/BR blends provide the better treadwear resistance performance particularly with a high BR content. NR also has the lower hysteresis response for lower rolling resistance in truck tires.
The performance improvements needed for TBS/RT include improved treadwear and the reduction of tread compound hysteresis. The necessary emphasis is to reduce tread damage during service, particularly due to hysteresis, treadwear and tear performances. To satisfy these demands, tire components must develop high tear strength, high tensile strength and low heat build-up properties. For these reasons, the art has recognized a long felt but unresolved need for a more reinforcing and less hysteretic N100 series tread black for use in truck tire tread compounds. (See, e.g., M. B. Rodgers and S. M. Mezynski, Kautschuk Gummi Kunst., 46, (9), 718 (1993)).
The present invention satisfies this long felt but unmet need and provides a carbon black in the N100 series which imparts upon tire tread rubber improved treadwear, lower rolling resistance, lower heat buildup and improved tear resistance. The improved carbon black is in the N100 series and, when in rubber, combines the long treadwear and low heat buildup advantages of N121 carbon black with the high tear resistance properties of N115. The present invention particularly applies to truck and bus steel cord radial tire treads, especially for use in on/off highway conditions, and for high performance passenger car tires.
The present invention relates to improved carbon black for tire tread rubber. More particularly, the present invention relates to carbon black which imparts upon tire tread rubber improved treadwear, lower rolling resistance, lower heat buildup and improved tear resistance. The present invention has particular application to truck and bus steel cord radial tire treads, especially for use in on/off highway conditions, and for high performance passenger car tires.
More particularly, the present invention relates to a carbon black in the N100 series, having a reduced structure, with a mean particle size of between about 16.0 and about 19.0 nm and a structure level, measured in DBPA of between about 100 to about 115 cc/100 g. More particularly, carbon black having a reduced structure, wherein the mean particle size is measured per ASTM D3849, procedure D and is between about 17.0 and about 18.0 nm. Still more particularly, a carbon black having a reduced structure, wherein the structure is measured by ASTM D2414 and is between about 105 and about 110 cc/100 g.
The present invention also relates to an improved carbon black having a modified aggregate shape distribution as determined by electron microscopy (See, e.g., C. R. Herd, et al., Rubber Chemistry and Technology 66, 491 (1993)) in conjunction with skeletonization of the aggregates via automated image analysis. Of the four shape categories defined for carbon black aggregates (1-Spheroidal, 2-Ellipsoidal, 3-Linear and 4-Branched), the present invention relates to an improved carbon black having a higher level of low structure ellipsoidal aggregates compared to conventional tread blacks N115 and N121. Further, it was found that the carbon black of the present invention has a lower number of volume-weighted aggregate branches about the mode of the volume-weighted distribution of branches. This narrower distribution about the mode is characterized by the DELTA B50, which is the full width at half maximum of the volume-weighted branch distribution.
The present invention further relates to an improved rubber composition containing an improved carbon black. More particularly, the present invention relates to an improved rubber composition containing an improved carbon black, wherein the carbon black imparts upon rubber composition improved treadwear, lower rolling resistance, lower heat buildup and improved tear resistance. The present improved rubber composition has particular application to truck and bus steel cord radial tire treads, especially for use in on/off highway conditions, and for high performance passenger car tires.
More particularly, the present invention relates to an improved rubber composition containing an improved carbon black in the N100 series, the improved composition having a treadwear rating for a natural rubber tread formulation on radial passenger cars of 107.2% relative to N110 at 100%, and tan delta hysteresis levels of 0.131 versus 0.137 and 0.145 for N100 and N115, respectively. The trouser tear resistance level of the improved composition is 82.6 kN/m compared to 81.3 and 49.3 kN/m for N115 and N121, respectively. More particularly, the improved rubber composition has abrasion and hysteresis properties of about 5%-10% higher and about 4 to 11% lower, respectively, and equivalent tear properties relative to N110 or N115. Still more particularly, the improved rubber composition has improved abrasion resistance (treadwear) and lower hysteresis properties of about 7% to about 10%, and about 5% to about 10%, respectively, as measured in relative treadwear ratings and tan delta level performances.
Further, the present invention relates to an improved rubber composition, having an improved carbon black with a reduced structure, in DBPA, of between about 105 to about 110 cc/100 g and improved tear resistance properties, comparable to N110 and N115, measured in kN/m, of between about 80 and about 85 kN/m.
The data herein describe a new tread black, carbon black A, that has been developed for application in truck/bus steelcord radial tire treads to address the specific needs for improved treadwear, lower rolling resistance, lower heat build-up and improved tear resistance. This tread black is classified as having an N100 fineness level, with a low surface microporosity level and a DBPA structure level comparable to N110 or N115.
In the experiments below, carbon black A was evaluated relative to conventional N100 and N200 series tread blacks in two separate truck tread formulations: NR and NR/BR (65/35). In the non-oil-extended NR formulation, carbon black A was compounded at a 50 phr loading level relative to N110, N115 and N121 at the same loading. In the oil-extended (25 phr) NR/BR formulation, carbon black A was compounded at a 60 phr loading level relative to N110, N115, N121 and N234. However, the tread blacks were preferentially located in the elastomer blends with more (49.5 phr) in the NR phase. This distribution was achieved through the preparation and subsequent blending of separate elastomer masterbatches.
In both tread formulations, carbon black A (relative to N110, N115 and N121) was shown to provide the desired specific performances of improved tear resistance, lower predicted rolling resistance, lower heat build-up and lower Mooney viscosity levels, while maintaining acceptable stress-strain and hardness property levels.
In treadwear testing in the NR tread formulation, carbon black A had better treadwear ratings than N110, with ratings equal to N121. For the NR/BR tread formulation, carbon black A exhibited better fatigue and better cut-growth resistances relative to N110, N115, N121 and N234. With respect to predicted relative wet traction performance, carbon black A was superior to N110, comparable to N115 and N234, and close to the performance level of N121 in the NR/BR formulation. The predicted dry traction was better than N121, similar to N234 and less than N110 and N115.
Although this invention relates to tread compounds containing NR or NR with BR, use of carbon black A in synthetic elastomers is expected to provide similar performance improvements. The unique performance of carbon black A with respect to N110, N115 and N121 suggests applicability to truck tire treads where more severe service demands are experienced as well as to off-the-road tire treads. Here, the good heat build-up, tear, fatigue and cut-growth properties will provide improved tire performances.
Other appropriate applications for carbon black A include high performance passenger tires, racing tires and truck tire retread compounds.